The purpose of these studies is to determine whether a subset of patients with breast cancer can be identified which is characterized by the presence in blood serum of antibody that reacts with mouse mammary tumor virus (MMTV). Lymphocytes of patients whose sera are positive for MMTV-related antibody by indirect immunofluorescence and enzyme linked immunoassay (ELISA) will also be typed for major histocompatability antigens (HLA). The data will be analyzed to determine if the presence of antibody that reacts with MMTV, is associated with certain HLA specificities. The data will also be evaluated to determine if the presence of MMTV-related antibody and/or certain HL-A specificities are associated differently among patients from breast cancer - prone families than among patients who do not have a family history of the disease.